15. General anaesthesia, injectable anaesthetics Flashcards
(30 cards)
What is general anaesthesia?
General anaesthesia is the complete loss of sensory, voluntary and reflexive motor activity, while keeping the respiratory and vasomotor centre functioning. Analgesia is NOT the same. We have general and local anaesthesia
- Local anaesthesia = Without loss of consciousness
- General anaesthesia = Total loss of consciousness
Definitions of analgesia and anaesthesia
Analgesia = Lack of pain sensation
Anaesthesia = Lack of sensation, total loss of motoric activity and pain relief
General anaesthesia, mechanism of action
1. Presynaptic action
In order for a nerve cell (neuron) to send a message, it needs to release neurotransmitters, which all depend on the calcium entering the cell through voltage-gated calcium channels. In general anaesthesia, the calcium channels are blocked and the neurotransmitter release is reduced.
2. Postsynaptic action
After neurotransmitters are released, they travel to the next nerve cell and bind to the receptors there. The drugs of general anaesthesia make the binding harder, we call this depression of excitatory neurotransmission.
At the same time, general anaesthesia can enhance the effect of GABA, which is an inhibitory neurotransmitter, by binding to a specific site on the GABAA receptor called allosteric activation, making it easier for GABA to inhibit the nerve cell (neuron)
Name the different stages of general anaesthesia
1. Stage 1
Analgesia begins, the induction phase. The patient experiences disordered consciousness
2. Stage 2
Excitement phase.
The patient might experience signs of delirium and agitated behaviour. It occurs as the patient transitions from consciousness to unconsciousness
3. Stage 3
Surgical anaesthesia. Characterised by unconsciousness, amnesia, immobility and unresponsiveness to surgical stimulation.
4. Stage 4
Overdose., leading to respiratory paralysis
Injectable anaesthesia drug groups
- Barbiturates
- Imidazole anaesthetics
- Propofol
- NMDA receptor antagonists
- Steroid anaesthetics
Active substances of barbiturates
- Pentobarbital
- Thiopental
- Methohexital
Active substances for Imidazole anaesthetics
- Metomidate
- Etomidate
Active substances for steroid anaesthetics
- Alfaxolane
- Alfadolone
Active substances for NMDA receptor antagonists
- Ketamine
- Tiletamine
Ketamine is a controlled drug due to it being a psychotropic drug, tiletamine is not a controlled drug
Mechanism of action of barbiturates
GABA neurotransmitters bind to GABA receptors and inhibit nerve impulses, leading to a calming effect.
There are secondary effects, such as the decrease in calcium accumulation in the neuron. They need calcium to release the neurotransmitters.
GABA receptors play a role in regulating brain activity, reducing anxiety and promoting relaxation.
Barbiturates work by attaching to a specific spot (the allosteric site) on the GABA A receptor. When they attach, they make the GABA A receptor work better, making the calming effect stronger
Pharmacological effect of Barbiturates
Affects CNS, circulation and respiration
Pharmacokinetics of barbiturates
pKa, lipophilicity, lipoid/water partition all affect how well the drug is absorbed into the body, how
Redistribution and metabolism
How the drug moves through the body
Administration of barbiturates
IV !!
Effective and harmless
Indication of barbiturates
- Induction
- General anaesthesia
- Epilepsy, pentobarbial IV
- Euthanasia
Contraindications for barbiturates
- Younger age ⭢ under 2 months
- Greyhounds are oversensitive !!
Side effects of barbiturates
- Respiratory depression
- Cardiovascular depression
- Tissue irritation
- Postnarcotic excitations ⭢ seen frequently
Mechanism of action of propofol
Binds to the GABAa receptor.
GABA is the neurotransmitter that reduces the activity of neurons in the brain.
In short: The drug helps to “calm” the brain by boosting the effects of the brain’s natural calming signals
Pharmacological effect of propofol
Unconsciousness
Muscle relaxation is good
Pharmacokinetics of propofol
-
Onset/duration: IV. The effect is quick, 30-45 seconds. It lasts for about 5-15 minutes with a single dose. Continuous infusion can extend the duration
Recorvery is quick and complete -
Distribution: Propofol distributes rapidly throughout the body, including the brain.
It can cross the blood-brain-barrier and the placenta. -
Metabolism: Quickly in the liver and extrahepatic sites, like the lungs
Propofol can safely be administered in liver failure. - Elimination: In urine
Indication of propofol
Induction and maintain general anaesthesia
Used as treatment for epileptic seizure, but this happens rarely
Contraindications for propofol
Cardiac and respiratory problems, hepatic impairment
Side effects for propofol
- Transient apnoea during induction ⭢ The patient stops breathing when the drug is first given, but only for a shorter time
- Cardiovascular depression
- Vomiting
- Excitation during recovery, rare!
- Allergy
- Increased ocular pressure
Mechanism of action of NMDA receptor antagonists
They block the activity of NMDA receptors in the brain, which allow signals to pass between nerve cells when they are activated. By blocking the receptors, NMDA antagonists can have various effects, such as pain relief, anaesthesia.
NMDA = N-methyl-D-aspartate
Side effecs of ketamine
- Catalepsy-like condition ⭢ The animal becomes rigid and unresponsive (like a statue)
- Increased cardiac output
- Increased heart rate
- Increased blood pressure
- Arrhythmias
- Hallucinations
- Slight increased salivation
